This invention relates to apparatus for measuring the thickness of a roll of web material such as photographic film, paper, textile or the like, winding on or unwinding from a core.
In winding/unwinding apparatus for winding a web onto a core and unwinding it therefrom, it is necessary, when winding, to make the total thickness of the wound roll uniform for providing wound rolls of uniform quality, and when unwinding webs while splicing them end to end by using an automatic splicer, to ensure that the ends left on the core will be as short as possible for economy. Therefore, such machines are provided with apparatus for measuring, say, the remaining thickness of a roll unwinding from a core, which controls a driving mechanism so as gradually to reduce the unwinding speed when it detects a remaining thickness of unwinding roll approaching a predetermined one, and to stop upon detecting the predetermined thickness.
For example, Japanese Patent Publ. 47-13292 discloses such apparatus as mentioned above, whereby the transport speed of a web unwinding from a master roll is measured. At the time when a predetermined transport speed of web is detected, the apparatus controls a driving mechanism to begin the reduction of its driving rate. During the reduction, the apparatus calculates the desired instantaneous transport speed of the web from the target remaining thickness of the unwinding roll, the present remaining thickness of unwinding roll, the reduction ratio of the driving rate and the thickness of the web, and controls the driving mechanism so as to make the transport speed of the web follow the desired transport speed thus calculated, in order that final thickness will accurately match the target thickness (i.e. the final remaining thickness of the roll on the core). Generally speaking, because the transport speed often fluctuates, an error in measuring the remaining thickness of the roll on a core can result from such fluctuation.
There has been proposed an apparatus independent of fluctuations in the transport speed of the web, which is disclosed in, for example, Japanese Patent Unexamined Publ. 51-31257. In such apparatus, there are provided pulse signals for every unit of transferred length of the web and for every fractional revolution of the core on which the web is rolled. Continuously changing diameter of wound web is detected by counting the pulses corresponding to the transferred length of web per unit fraction of a revolution of the core.
Both devices mentioned above have serious disadvantages, one being that noticeable error appears easily in the measurement of the thickness of the web due to the fact that the core is subject to eccentric rotation because the core and the chucks rotatably supporting it at both ends are not coaxial and/or the core is eccentric itself.